LARTC - Nov 2003 - Traffic shaping and MTU/MSS

Hi,

I''m using traffic shaping on my home 1.5M/256K ADSL connection. The
problem
I''m running into is even when I''m shaping upload traffic (web
server/p2p
etc...) I run into latency issues with online gaming. This is of course due 
to the 256Kb upload and the 1492 MTU (PPPoE) which means a single large 
packet will take up to 40ms to transfer. 

What I would like to do is reduce the MSS for low priority traffic using 
ipchains based on a fwmark value. Is it possible to do this? 

Appreciate any other suggestions to solve this problem.

Thanks.

Marc
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Hi Marc,
> What I would like to do is reduce the MSS for low priority traffic using 
> ipchains based on a fwmark value. Is it possible to do this? 
ipchains, or iptables?
you should be able to do it with a single rule with iptables:

iptables -A OUTPUT -p tcp --syn -m mark --mark XX -j TCPMSS --set-mss XXX

although i''ve never done this myself...

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On Monday 24 November 2003  7:35 am, Defekt wrote:
> Hi,
>
> I''m using traffic shaping on my home 1.5M/256K ADSL connection.
The
> problem I''m running into is even when I''m shaping upload
traffic (web
> server/p2p etc...) I run into latency issues with online gaming. This is
> of course due to the 256Kb upload and the 1492 MTU (PPPoE) which means a
> single large packet will take up to 40ms to transfer.
>
> What I would like to do is reduce the MSS for low priority traffic using
> ipchains based on a fwmark value. Is it possible to do this?
>
> Appreciate any other suggestions to solve this problem.
One thing to consider - assuming things are the same in your country as 
mine, is that DSL modems are atm devices an send whole 53 byte cells. So 
MTU = x+1 may send 53 bytes more than MTU = x, so choose carefully. I 
can''t tell you exactly how to see what is right for you as I use pppoa,
I
think the overhead on pppoe is either 32 or 40 bytes, so you would find an 
MTU (MSS+40) add say 32 then divide by 48 (as each 53 byte cell carries 48 
data) and change MTU until you get a whole number.

If you can get your modem to give you a cell count you could test what the 
correct pppoe overhead is with ping -s x you could change x until the cell 
counter varied by 1 between x and x+1. If the overhead is 32 then ping -s 
36 should send 2 cells (36 data + 8 ICMP + 20 IP +32 PPP = 96) & 37 three, 
s = 28 for two cells / 29 for three if it''s 40.

Andy.


Andy.

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